Shock-resistant wearable pH sensor based on tungsten oxide aerogel
Chen-Xin Wang , Guang-Lei Li , Yu Hang , Dan-Feng Lu , Jian-Qi Ye , Hao Su , Bing Hou , Tao Suo , Dan Wen
The sweat wearable biochemical sensors (WBS) can obtain the biochemical index information in human body via a non-invasive way [1, 2], where the collected information is transmitted to the intelligent electronic devices via Bluetooth, near field communication (NFC) or Wi-Fi technology [3]. Thus, the WBS are considered as ideal miniature devices for real-time, out-of-clinic health monitoring [4-6]. Among various sensing strategies, the electrochemical approach has been widely used for the biomarker monitoring due to its simple design, high sensitivity, easy integration and low cost [7-9]. For example, Cheng et al. reported an electrochemical biosensor based on the stretchable gold fibers, which exhibited a high sensitivity and a good stretchability in the pH detection [10]. Zhang et al. developed a robust integrated wearable electrochemical sensor for the detection of cortisol, Mg2+, and pH in human sweat with low detection limits and high sensitivities [11]. Despite significant progresses in the electrochemical WBS, their performance under extreme conditions, such as shockwave, has been rarely explored. However, it is crucial to ensure stable monitoring of biochemical indicators, providing early out-of-clinic information for blast injuries [12].
The sensing performance of the electrochemical WBS depends primarily on the physicochemical stability of the sensing electrodes [13]. In this sense, the exploration of the sensing materials with favorable electrochemical activity as well as excellent shock-resistance is promising. The rise of nanomaterial science provides opportunities for developing highly stable electrochemical WBS under shockwave environments [14-17]. As a kind of new-emerging nanomaterials, metal-based aerogels consist of interconnected networks possesses unique physical and chemical properties (e.g., open interconnected porosity, large specific surface area, and high electrocatalytic activity) [18-20]. These properties endow them with superior sensing performance when applied as sensing materials [21, 22]. Particularly, these aerogels with self-supported architectures, flexibility and self-healing are expected to exhibit remarkable structural stability by preventing their aggregation in extreme environments [23, 24]. Consequently, metal-based aerogels hold great potentials for constructing highly stable electrochemical WBS.
As we known, the pH value is an important clinical pathological parameter which can provide the early monitoring for human body injuries [25-28]. Meanwhile, metal oxides, especially nanostructured tungsten oxide (TO), have been widely used as a pH-sensitive layer due to their chemical activity and stability, low cost, biocompatibility, and high selectivity against common interfering ions [29, 30]. Herein, in this work, a TO aerogel (TOA) synthesized by a hydrothermal method was successfully applied to construct a sensitive and stable wearable pH sensor. The continuous metal oxide framework in the TOA can accelerate the mass transfer diffusion and electron transfer, ensuring high sensing performance. The developed wearable pH sensor exhibited high sensitivity, good selectivity and reproducibility, and superior stability in the range of 3–8. When used for non-invasive and real-time pH monitoring on human skin, the very small relative deviation (1.91%) from the results of the commercial pH meter verified the reliability of the as-fabricated wearable pH sensor in practical application. Furthermore, the wearable pH sensor displayed an outstanding shock-resistance (118.38 kPa) with negligible influence on the sensitivity, primarily due to the self-supported architectures and flexibility of the TOA.
The TOA was synthesized by a hydrothermal method, as illustrated in Fig. S3 (Supporting information). A dark blue oxide gel was obtained by in situ growth of tungstic acid and polyethyleneimine under acidic condition (pH 3). Figs. 1A and B show photographic images of the TO hydrogel, which exhibited a cylindrical appearance and maintained its intact appearance even under applied weight. After freeze drying, the resulting TOA was extremely light and could stand steadily on the top of leaf, suggesting its ultra-low density and extremely high porosity (Fig. 1C). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted to investigate the microstructures of the TOA. As can be seen from Fig. 1D, the TOA exhibited a loose three-dimensional (3D) porous structure, providing large specific surface area and abundant active sites. The TEM image (Fig. 1E) evidently showed that the TOA, with an interconnected network, was composed of the ultrathin nanowires with numerous bifurcations (9.3 ± 1.8 nm in diameter, Fig. S4 in Supporting information). This endowed the TOA with good flexibility, which could improve the structural stability when subjected to the shock wave environment. The lattice spacing of the TOA was measured to be 0.374 nm from Fig. 1F, consistent with the theoretical value of the (010) facets of W18O49 [31]. According to the X-ray diffraction (XRD) pattern (Fig. 1H), the diffraction peaks at 23.5°, 26.3°, 34.2°, 48.8° and 54.4° corresponded to the (010), (−104), (−512), (021) and (123) facets of W18O49 [PDF# 05–0392], indicating that the main component of the as-prepared TOA was W18O49. Additionally, energy dispersive X-ray spectroscopy (EDS) results confirmed a similar atomic ratio of 0.38 for tungsten and oxygen elements, which agreed with the XRD pattern and further implying the successful preparation of W18O49 (Fig. 1G).
The surface chemistry of the TOA was investigated using X-ray photoelectron spectroscopy (XPS). As recorded by the full-scan XPS spectrum (Fig. S5 in Supporting information), both W and O elements appeared in the as-prepared TOA. Fig. 1I presents the high-resolution XPS spectrum of tungsten, revealing two pairs of characteristic peaks at 34.24 eV and 36.34 eV, as well as 33.14 eV and 35.04 eV, corresponding to W 4f7/2 and W 4f5/2 of W6+ and W5+, respectively. Additionally, the characteristic peak at 528.94 eV in Fig. 1J was assigned to the W-O bond, suggesting the formation of TO in the aerogel [32, 33]. The specific surface area and porosity of the TOA were acquired from the Nitrogen adsorption-desorption measurement. As shown in Fig. 1K, the nitrogen adsorption-desorption isotherms of the TOA displayed type II and type IV curves. The specific surface area of the TOA was calculated to be 116.21 m2/g, which is higher than those of other TO materials with porous structures [34, 35], probably providing plentiful active sites for the further potentiometric pH sensing. And the pore size and pore volume distribution results in Fig. 1L indicated the presence of micropores and mesopores. Combining the plenty macropores observed in the SEM image, the TOA revealed a hierarchical porous structure with 3D interconnected networks and a wide range of the pore size distribution.
It is well documented that the TO nanomaterials have been intensively studied in pH sensing for their easy availability, reversible change of conductivity, high selectivity and good biocompatibility [36]. Herein, the pH response of the TOA modified glassy carbon electrode (TOA/GCE) was elucidated through the cyclic voltammetry (CV) in the pH range of 3–8, which covers the possible variation of the pH value in human sweat (Fig. 2A). Since the TOA is insulating when oxidized and conducting when reduced, the formation of its conductive state is related to the proton embedding in aqueous solutions [37]. The increased cathodic current below 0 V and its shift to more positive potential at acidic pH indicated the intercalation of protons, forming conductive tungsten bronze. This phenomenon could be attributed to the protons inserted into the TO crystals in the acid solution, which is the basis of the potentiometric pH sensing [38, 39].
The protons can reversibly interact with the TOA, leading to changes in the response voltage, which were measured via the open circuit potential-time (OCPT) curve. As depicted in Fig. 2B, the potentials decreased with the increasing pH values, with a detectable limit of 0.05. Further numerical fitting in Fig. 2B yielded a linear response (R2 = 0.99) ranging from pH 3 to 8 (Fig. 2C). Controlled experiments were also carried out on the TONP modified GCE (TONP/GCE) for comparison. The TOA-based pH responsive electrode exhibited 3 times' higher sensitivity than that of the TONP/GCE (Fig. S6 in Supporting information). This enhanced sensitivity can be ascribed to the aerogel structure of the TOA, which provides large specific surface, numerous exposed active sites, and highly efficient channel for protons [21, 24]. The selectivity of the pH sensing electrode was confirmed by very little potential changes observed after adding interfering ions in Fig. 2D and Fig. S7 (Supporting information), including 1 mmol/L NH4+ (0.45%), 8 mmol/L KCl (0.61%), 20 mmol/L NaCl (0.75%) and 1 mmol/L CaCl2 (1.90%). Moreover, the TOA/GCE demonstrated an excellent repeatability with a relative standard deviation (RSD) less than 3% (n = 3, Fig. 2E, Fig. S8 and Table S1 in Supporting information). Furthermore, a very slight variation in the response potential (1.98%) after 50000 s' test indicated a good sensing stability of the TOA-based pH electrode, as shown in Fig. 2F. Moreover, the ambient temperature had little influence on the pH sensing performance of the TOA-based pH electrode (Fig. S9 in Supporting information).
Next, the TOA was modified onto a flexible screen-printed electrode (TOA/SPE) to serve as the sensing chip for the pH sensor. The cathodic current increased below 0 V as the pH value decreased (Fig. S10 in Supporting information), similar as the behavior observed on the GCE, demonstrating the feasibility of the sensing chip for wearable pH sensing. The sensitivity of the sensing chip within a linear pH range of 3–8 in 0.1 mol/L phosphate buffer solution (PBS) was calculated to be −63.70 mV/pH (Fig. S11 in Supporting information). This is close to the theoretical Nernstian value (−59.1 mV/pH) and superior to those tungsten oxide (TO) based pH sensing electrodes reported previously (Table S2 in Supporting information). The negligible changes of the RSD among three different tests on the same sensing chip indicated an excellent repeatability (Fig. 3A, Table S3 in Supporting information) and reversibility (Fig. S12 in Supporting information). Furthermore, we evaluated the electrochemical performance of the sensing chips in the artificial sweat. The response potential was associated with the different pH of the artificial sweat in the range of 3–8, showing a good linear correlation of 0.99 (Figs. 3B and C).
Considering the excellent sensing performance, a wearable pH sensor based on the TOA/SPE was successfully constructed by integrating it with a signal processing circuit. The calibration curve (Fig. 3E) measured by the signal processing circuit in the artificial sweat facilitated the wearable application for pH monitoring on human skin (Fig. 3D). The monitoring data transmitted to a mobile phone via a Bluetooth module (pH 6.68, n = 3) are shown in Fig. 3F. The very small deviation (1.91%) between this value and that obtained from a commercial pH meter (pH 6.81, Fig. 3G) verifies the reliability of the wearable pH sensor in practical application (Fig. 3H).
In order to demonstrate the feasibility for the real-time detection in the blast injury, we evaluated the shock resistance of the TOA-based pH sensor. A shock tube was employed to generate the shock wave by compressing the air, resulting in a rapid rise in pressure from atmospheric pressure to peak overpressure [40, 41]. As shown in Fig. S13 (Supporting information), the shock wave overpressure was measured to be 118.38 kPa. The hierarchically 3D porous microstructures of the TOA remained almost unchanged before (Fig. 4A) and after (Fig. 4B) the impact due to its unique structure features of crosslinked nanowire networks, suggesting a good structural stability of the TOA. Meanwhile, the sensing performance of the TOA/GCE only showed a slight change (5.67%) in sensitivities toward pH before and after the impact, indicating a superior electrochemical stability (Fig. 4C). The anti-impact behavior of the TOA-based sensing chip was further evaluated. One can easily see the good morphologic stability without any observable changes (Figs. 4D and E) before and after the impact. Moreover, the shock wave produced a small effect on the sensitivity (7.17%) of the sensing chip in the pH range of 3–8, before and after the impact, demonstrating an excellent sensing stability (Fig. 4F). This remarkable shock-resistance of the TOA-based pH sensing electrode was probably benefited from its self-supported architectures, flexibility and self-healing characteristics [23].
We further evaluated the sensing performance of the wearable pH sensor before and after the impact by conducting wearable experiments on a pig (Fig. S14 in Supporting information). The results in Figs. 4G and H showed that the pH values displayed via a mobile phone in the artificial sweat was 6.22 and 6.19 (n = 3), respectively, before and after the impact. The value obtained using a commercial pH meter was 6.10 (n = 3, Fig. S15 in Supporting information). A very little variation (0.50%) implied that the TOA-based sensor could be applied for pH monitoring under the impact conditions, offering great potentials for the early detection of impact injuries (Fig. 4I). The experiments involving animal/human subjects were guided by the ethical regulations under a protocol (Nos. 202201171 and 202202059) which was approved by the Medical and Laboratory Animal Ethics Committee of the NPU.
In this work, we successfully constructed a wearable pH sensor using a tungsten oxide aerogel-based sensing electrode combined with a signal processing circuit, enabling pH monitoring even under blast wave conditions. The electrochemical results showed a high sensitivity (−63.70 mV/pH), a good stability (over 50, 000 s), and a superior reproducibility in the pH range of 3–8. With its remarkable sensing performance, the wearable pH sensor realized the non-invasive and real-time sweat pH detection with minimal relative deviation (1.91%) from the results of a commercial pH meter. Importantly, the sensor showed outstanding stability even after impact (118.38 kPa), demonstrating its suitability for real-time monitoring under extreme conditions. Overall, our work provides a simple yet effective model for sensing electrode materials and wearable electrochemical sensors for the real-time monitoring under impact conditions.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Chen-Xin Wang: Writing – original draft, Investigation, Data curation. Guang-Lei Li: Writing – review & editing, Investigation. Yu Hang: Investigation. Dan-Feng Lu: Investigation. Jian-Qi Ye: Investigation. Hao Su: Investigation. Bing Hou: Supervision, Conceptualization. Tao Suo: Supervision, Conceptualization. Dan Wen: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.
This work was supported by the National Natural Science Foundation of China (Nos. 22374119 and 22274127), the Research Fund of the State Key Laboratory of Solidification Processing (NPU), China (No. 2021-QZ-01), the Key Project of Natural Science Fund of Shaanxi Province (Nos. 2023-JC-ZD-06 and 2024JC-YBQN-0636), and the Open Project of the State Key Laboratory of Transducer Technology (No. SKT2307).
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 The optical photos of (A, B) the TO hydrogel and (C) aerogel. (D) The SEM, (E) TEM, (F) HRTEM and (G) EDS images of the TOA. (H) XRD pattern of the TOA. (I, J) XPS spectra of W 4f and O 1s. (K) The nitrogen adsorption-desorption isotherms and (L) pore-size distribution of the TOA.
Figure 2 (A) CVs of the TOA/GCE in the pH range of 3–8 with a scan rate of 50 mV/s. (B) The OCPT and (C) calibration curve of the TOA/GCE in the pH range from pH 3 to 8. (D) The selectivity of the TOA/GCE in 0.1 mmol/L phosphate buffer solution (PBS) with 1 mmol/L NH4+, 8 mmol/L KCl, 20 mmol/L NaCl, and 1 mmol/L CaCl2. (E) The repeatability of the TOA/GCE during three cycles for pH range of 3–8. (F) The operating stability of the TOA/GCE in 0.1 mol/L PBS (pH 6).
Figure 3 (A) The repeatability of the TOA/SPE in 0.1 mol/L PBS. (B) The OCPT curves and (C) the corresponding calibration curve of the TOA/SPE in the artificial sweat. Wearable pH sensor: (D) the schematic illustration of the application experiment, (E) the calibration curve measured in the artificial sweat by the signal amplifier circuit, (F) the real-time pH monitoring via a mobile phone, (G) the pH value measured via a commercial pH meter, and (H) the comparison of the results of the TOA-based wearable sensor with a commercial pH meter.
Figure 4 (A, B) The SEM images of the TOA and (C) the corresponding calibration curves of the TOA/GCE before and after the impact. (D, E) The optical photos of the sensing chips and (F) their corresponding calibration curves before and after the impact. pH monitoring of the artificial sweat by the wearable sensor (G) before and (H) after the impact, and (I) the comparison of the results with a commercial pH meter.
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